1. Field of the Invention
The present invention relates to an optical attenuator, and more particularly, to a variable optical attenuator for attenuating the amount of the light transmitted through an optical fiber variably. The present application is based on Korean Patent Application No. 2001-61043, filed Sep. 29, 2001, which is incorporated herein by reference.
2. Description of the Related Art
As optical communication systems using optical fibers draw more attention, technologies related to optical communication devices or elements widely used in communication networks are under development proactively. As one of the optical communication elements, an optical attenuator attenuates a certain amount of the incident light and outputs the attenuated light. The optical attenuator includes a fixed optical attenuator which attenuates a fixed amount of the light, and a variable optical attenuator which attenuates a variable amount of the light. The optical attenuator is used when the light transmission factors vary depending on the optical communication elements. For example, in wavelength division multiplexing (WDM) systems which use lights with different wavelengths, the systems are designed to use the same intensities of the lights while laser beams used as light sources have different intensities. The optical attenuator compensates for the different light intensities. In addition, because channels in an optical switch or a multiplexer/demultiplexer of the WDM system have different light transmission factors, the optical attenuator makes the different light transmission factors identical. Especially, the variable optical attenuator is necessary to change the light transmission factors since different optical paths cause different intensities of light loss.
To satisfy the needs for the above applications, various types of optical attenuators have been developed. FIG. 1 illustrates one type of optical attenuator. The optical attenuator shown in FIG. 1 adopts Graded-index (GRIN) rod lens 12 and 22, and an optical shutter 25.
With reference to FIG. 1, the light entering through the optical fiber 11 of the input end is expanded and output by the input GRIN rod lens 12. The width of the light is expanded to about 10 μm˜1 mm or more. The expanded light output by the input GRIN rod lens 12 is input to the output GRIN rod lens 22 which faces and is separated from the input GRIN rod lens by a certain gap. The light is converged within the output GRIN rod lens 22 and output through the optical fiber 21 of the output end. On the optical path, an optical shutter 25 designed to attenuate the intensity of the light is positioned in the space 23 between the two GRIN rod lenses 12 and 22 and intercepts some of the progressing light. The optical shutter 25 moves within the space 23 under the control of the actuator 27 so that it can adjust the amount of the intercepted light, and the controller 29 controls the movement distance of the optical shutter 25.
As described above, the existing variable optical attenuator includes the GRIN rod lenses 12 and 22. Since the GRIN rod lenses can act as collimators, the output light does not spread and becomes parallel light. Therefore, however long the distance between the GRIN rod lenses is, the light loss can be minimized in the space 23. Moreover, because the GRIN rod lenses 12 and 22 can expand the width of the light to 1 mm or more, the movement distance of the optical shutter 25 may be long. If only the optical fibers 11 and 21 are used without the GRIN rod lenses 12 and 22, too narrow width of the light necessitates the adjustment of the optical shutter 25 with a precision of 0.1 μm or less. It is very difficult to achieve the precision and guarantee the reliability. On the contrary, if the GRIN rod lenses 12 and 22 are used, a separate device is necessary to align the GRIN rod lenses precisely. Generally, an active align device is used to assemble the GRIN rod lenses in the optimized condition so that the light can be transmitted from the optical fiber 11 to the optical fiber 21. That is the reason why the optical attenuator having the GRIN rod lenses 12 and 22 is not suited for mass-production.
FIG. 2 illustrates another type of existing optical attenuator. The optical attenuator shown in FIG. 2 adopts a waveguide 31 instead of the GRIN rod lens, and is applied to an arrayed waveguide grating (AWG) which is a WDM demultiplexer. That is, only a heater 33 needs to be installed on the waveguide of the optical communication element to implement the optical attenuator shown in FIG. 2. As for the optical attenuator having such configuration, when the heater 33 heats the waveguide 31, the characteristics of the light transmission of the waveguide 31 is changed and as a result, the transmission factors of the light can be adjusted.
When compared to the optical attenuator shown in FIG. 1, the existing variable optical attenuator shown in FIG. 2 features a simpler structure and a more streamlined manufacturing process, and can be mass-produced in array forms. However, the optical attenuator shown in FIG. 2 responds to changes in the ambient temperature sensitively in that the heater heats the characteristics of the light transmission. That is, changing ambient temperatures surrounding the element causes the characteristics of the waveguide 31 to be changed and further may lead to unnecessary change of the characteristics. Therefore, a temperature adjustment device is required to calibrate temperatures.